El Niño and La Niña are opposite phases of what is known as the El Niño-Southern Oscillation (ENSO) cycle. The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific (approximately between the International Date Line and 120 degrees West).
La Niña is sometimes referred to as the cold phase of ENSO and El Niño as the warm phase of ENSO. These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate.
El Niño and La Niña episodes typically last nine to 12 months, but some prolonged events may last for years. They often begin to form between June and August, reach peak strength between December and April, and then decay between May and July of the following year. While their periodicity can be quite irregular, El Niño and La Niña events occur about every three to five years. Typically, El Niño occurs more frequently than La Niña.
El Niño means The Little Boy, or Christ Child in Spanish. El Niño was originally recognized by fishermen off the coast of South America in the 1600s, with the appearance of unusually warm water in the Pacific Ocean. The name was chosen based on the time of year (around December) during which these warm waters events tended to occur.
The term El Niño refers to the large-scale ocean-atmosphere climate interaction linked to a periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific.
Typical El Niño effects are likely to develop over North America during the upcoming winter season. Those include warmer-than-average temperatures over western and central Canada, and over the western and northern United States. Wetter-than-average conditions are likely over portions of the U.S. Gulf Coast and Florida, while drier-than-average conditions can be expected in the Ohio Valley and the Pacific Northwest.
La Niña means The Little Girl in Spanish. La Niña is also sometimes called El Viejo, anti-El Niño, or simply “a cold event.”
La Niña episodes represent periods of below-average sea surface temperatures across the east-central Equatorial Pacific. Global climate La Niña impacts tend to be opposite those of El Niño impacts. In the tropics, ocean temperature variations in La Niña also tend to be opposite those of El Niño.
During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest.
Sea surface temperature in the equatorial Pacific Ocean (above). El Niño is characterized by unusually warm temperatures and La Niña by unusually cool temperatures in the equatorial Pacific. Anomalies (below) represent deviations from normal temperature values, with unusually warm temperatures shown in red and unusually cold anomalies shown in blue.
To filter out month-to-month variability, average sea surface temperature in the Niño 3.4 region is calculated for each month, and then averaged with values from the previous month and following month. This running three-month average value is compared with average sea surface temperature for the same three months during 1971 – 2000. The departure from the 30-year average of the three-month average is known as the Oceanic Niño Index or ONI.ENSO Conditions
For real-time monitoring and prediction, NOAA considers El Niño conditions to be present when the Oceanic Niño Index is at least +0.5. In other words, El Niño conditions exist when the three-month average sea surface temperature in the Niño 3.4 region is at least 0.5°C warmer than average.
Conversely, NOAA declares that La Niña conditions exist when the Oceanic Niño Index is less than -0.5. This means that the three-month sea surface temperature in the Niño 3.4 region is at least 0.5°C cooler than average.
Whenever the Oceanic Niño Index is between +0.5 and -0.5, conditions are ENSO-neutral. A table of ONI values for each three-month period from 1950 to present is available from NOAA’s Climate Prediction Center.
El Niño and Its Impact on the United States
By examining seasonal climate conditions in previous El Niño years, scientists have identified a set of typical impacts associated with the phenomenon. Associated with doesn’t mean that all of these impacts happen during every El Niño episode. However, they happen more often during El Niño than you’d expect by chance, and many of them have occurred during many El Niño events.
In general, El Niño-related temperature and precipitation impacts across the United States occur during the cold half of the year (October through March). The most reliable of these signals (the one that has been observed most frequently) is wetter-than-average conditions along the Gulf Coast from Texas to Florida during this 6-month period. This relationship has occurred during more than 80% of the El Niño events in the past 100 years.
El Niño and Southeastern Wisconsin in 2015
These events can alter wind and weather patterns thousands of miles away. For us, El Niño brings a greater chance of warmer than normal temperatures with less snow over winter. As we approach the end of 2014 and are looking towards Winter weather in early 2015, it appears that there will probably be at least a weak El Niño effect. This does not necessarily mean warmer weather and less snow are a certainty as other weather phenomena can overshadow El Niño.
Recent strong El Niño events have been associated with the following impacts on weather in the Milwaukee area:
As indicated, we are on track for a weak El Niño event. A weaker El Niño would be expected to have a lesser effect on temperature and precipitation and right now it looks like an equal chance of it providing us with a warmer or a cooler winter.
Source: NOAA National Ocean Service.